1. Technical Field
This application relates to beamforming technologies that may be used in connection with portable ultrasound systems.
2. Description of Related Art
In recent years, a segment of the ultrasound community has focused on making ultrasound systems which are smaller, cheaper, and more power-efficient, while maintaining good image quality. These hand-held systems can become “ultrasonic stethoscopes” that allow physicians to perform ultrasound examinations almost anywhere and at anytime. See J. R. T. C. Roelandt, “Ultrasound stethoscopy: A renaissance of the physical examination?,” Heart, vol. 89, pp. 971-974, 2003; J. Hwang, J. Quistgaard, J. Souquet, and L. A. Crum, “Portable ultrasound device for battlefield trauma,” Proc. IEEE Ultrason. Symp., vol. 2, pp. 1663-1667, 1998; U. Rosenschein, V. Furman, E. Kerner, I Fabian, J Bernheim, and Y. Eshel, “Ultrasound imaging-guided noninvasive ultrasound thrombolysis: preclinical results,” Circulation, vol. 102, no. 2, pp. 238-45, 2000.
For example, a hand-held system can provide point-of-care diagnosis in remote locations, battlefields, emergency rooms, and private clinics. It can also be used in trauma or minimally invasive ultrasound-guided procedures, such as central catheter insertion. See J. Hwang, J. Quistgaard, J. Souquet, and L. A. Crum, “Portable ultrasound device for battlefield trauma,” Proc. IEEE Ultrason. Symp., vol. 2, pp. 1663-1667, 1998; U. Rosenschein, V. Furman, E. Kerner, I Fabian, J Bernheim, and Y. Eshel, “Ultrasound imaging-guided noninvasive ultrasound thrombolysis: preclinical results,” Circulation, vol. 102, no. 2, pp. 238-45, 2000. With wide-ranging applications in clinics, developing countries, and the military, the demand for portable ultrasound systems has increased rapidly in the last decade. The P10 from Siemens and the VSCAN from GE Healthcare are examples of recently introduced pocket-size ultrasound systems.
There have been many approaches to improving the size, cost, and quality of portable ultrasound systems. These include improvements in transducer design, transmit and receive circuitry design, and beamforming algorithms.
A significant percentage of the size and power of an ultrasound system can be devoted to the beamformer, which can be responsible for focusing the ultrasound beam both during transmit and receive operations. One standard beamformer consists of 64 to 128 transmit/receive channels. However, these can require expensive and bulky beamformers.
Beamforming
Beamforming approaches can be broadly categorized into two methods: analog beamforming and digital beamforming. In analog beamforming, the images may be formed by a sequence of analog signals which are delayed with analog delay lines, summed in the analog domain, and then digitized. In digital beamforming, the images may be formed by sampling analog signals from individual array elements, applying digital delays, and then summing digitally. See B. D. Steinberg, “Digital beamforming in ultrasound,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 39, no. 6, pp. 716-721, 1992.
Digital beamforming may apply time delays to focus digitized data (traditional delay-and-sum beamforming (DAS)) or may combine digital time delays with complex phase rotation. See M. O'Donnell, W. E. Engeler, J. J. Bloomer, and J. T. Pedicone, “Method and apparatus for digital phase array imaging,” U.S. Pat. No. 4,983,970, 1991; A. Agarwal, F. K. Schneider, Y. M. Yoo, and Y. Kim, “Image quality evaluation with a new phase rotation beamformer,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 55, no. 9, pp. 1947-1955, 2008. In digital DAS beamforming, the focused data may be summed and passed on for envelope detection and further signal processing. This method may be straightforward and intuitive, but may require a significant amount of hardware and processing capability.
Digital beamforming may also be achieved by digitizing the incoming RF data from each element, but then applying a coarse delay, followed by complex demodulation with phase rotation for fine delay. This method may reduce the computational demands compared to DAS beamforming, but may still require redundant hardware for each channel. See M. O'Donnell, W. E. Engeler, J. T. Pedicone, A. M. Itani, S. E. Noujaim, R. J. Dunki-Jacobs, W. M. Leue, C. L. Chalek, L. S. Smith, J. E. Piel, R. L. Harris, K. B. Welles, and W. L. Hinrichs, “Real-time phased array imaging using digital beam forming and autonomous channel control,” Proc. IEEE Ultrason. Symp., 1990, pp. 1499-1502.
A hybrid approach to beamforming may also be possible where different parts of the beamforming process occur in either analog or digital domains. In one case, echoes from clusters of elements may be delayed and summed in the analog domain and then digitized by a single analog-to-digital (A/D) converter. This approach may reduce the number of A/D converters compared to a full digital beamformer. See P. Pesque and J. Bouquet, “Digital ultrasound: from beamforming to PACS,” MedicaMundi, vol. 43, issue 3, pp 7-10, 1999.
To minimize the cost, power, and size of the beamformer, low-cost beamforming approaches have been proposed in the literature. One concept is the direct-sampled I/O (DSIQ) beamforming algorithm in which I/O data may be acquired by directly sampling the Q data one quarter-period after the I data. See K. Ranganathan and W. F. Walker, “Direct sampled I/O beamforming for compact and very low-cost ultrasound imaging,” IEEE Trans. Ultrason., Ferroelect., Freq. Control, vol. 51, no. 9, pp. 1082-1094, 2004. The DSIQ algorithm may rely solely on phase rotation of the I/O data to provide focusing. The proposed realization may use only one transmitter with a lower sampling rate and one I/O channel for each element. When used with a 2-D array, the DSIQ method may acquire C-scan images at 43 frames per second, B-scans with arbitrary plane orientation, and 3D images. See M. Fuller, K. Owen, T. Blalock, J. Hossack, W. Walker, “Real-time imaging with the Sonic Window: A pocket-sized, C-scan, medical ultrasound device,” Proc. IEEE Trans. Ultrason., Ferroelect., Freq. Control. Symp., 2009.
Another concept is known as Fresnel focusing. Originally used in optics, a Fresnel lens may be much thinner and therefore lighter than a conventional lens with the same focal point. In acoustics, physical Fresnel lenses were fabricated to focus ultrasonic waves for an acoustic microscopy system, which may provide high efficiency and focusing power, as well as a simpler manufacturing process. See B. Hadimioglu, E. G. Rawson, R. Lujan, M. Lim, J. C. Zesch, B. T. Khuri-Yakub, and C. F. Quate, “High-efficiency Fresnel acoustic lenses,” in IEEE Ultrason. Symp., pp. 579-582, 1993, S. C. Chan, M. Mina, S. Udpa, L. Udpa, and W. Lord, “Finite Element Analysis of Multilevel Acoustic Fresnel Lenses,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 43, no. 4, pp. 670-677, 1996. These physical lenses may replace the spherical focus of a conventional lens with an equivalent phase shift. Used with an array transducer, the Fresnel focusing technique may reduce the number of delays needed since different elements may require the same delay. These elements may then be clustered together.
In 1980, Fink first proposed the use of Fresnel focusing in an array-based system. In his experiment for 8-state Fresnel focusing, 8 different delays were used for transmit mode, while four different delays plus inverting amplifiers were used for receive mode. By imaging a 0.3 mm diameter copper wire, he showed that for a linear array, finer Fresnel phase sampling may lower the sidelobe level and the lateral resolution improves as f# decreases. See B. Richard, M. Fink, P. Alais, “New arrangements for Fresnel focusing,” in Acoustical Imaging, vol. 9, K. Wang, Ed. New York: Plenum Press, 1980, pp. 65-73.